Known seals serving the above-given purposes are normally formed by rings of toroidal shape or of rings which are simply of U-section without any filling serving to distribute pressure. When pressure-distributing blocks are used they are usually combined with U-section rings or with rectangular antiextrusion rings and fill the entire base of the groove or the region near the wall where the extrusion risk exists. There are also known seals which use U-section rings with a separate pressure distributing body that is not bonded to the U-section ring. This latter arrangement has the possibility that residual pressure builds up in a manner impossible in integral or bonded-together assemblies. Nonetheless all these seals have a symmetrical section which prevents them from functioning well at all pressures, particularly low pressure, and in addition these seals tend to have short service life even when provided with antiextrusion rings.